1. Field of the invention
This invention relates, generally, to a portable, battery-operated AC electromagnetic yoke for detecting surface defects in ferrous materials. More particularly, it relates to a circuit that produces a quasi AC square wave voltage so that the inspection is performed by the required method of common practice and is capable of demagnetizing upon completion of inspection.
2. Description of the prior art
There are a number of patents on AC electromagnetic yokes having utility in the field of magnetic particle inspection, but most are reliant upon common 115/230VAC alternating line current and therefore cannot be used in many field applications where 115/230VAC line current is not available or where there may be safety concerns associated with 115/230VAC line current. For example, see U.S. Pat. No. 2,136,375 to DeForest.
There are also patents on battery-powered electromagnetic yokes suitable for use in the field, but they produce DC currents which are not acceptable for most inspection requirements of common practice. Accordingly, these devices lack utility in applications where the inspected part must be in a demagnetized state at the completion of the inspection. U.S. Pat. No. 4,950,989 to Jones discloses a magnetic particle inspector that can use AC or half-wave DC.
Circuits for changing DC to AC, known as inverter circuits, are well known. However, electromagnetic yokes for magnetic particle inspection have inductors that produce large inductances and the power requirements of such yokes are high. Thus, common inverter circuits fail to provide the needed fill, square wave that closely emulates an AC wave.
U.S. Pat. No. 2,447,899 is believed to be the most relevant prior art patent because it discloses an electromagnetic particle inspector unit having a battery-powered circuit for producing an emulated AC voltage. However, the emulated AC voltage is not a true square wave and the device disclosed in that patent cannot generate a magnetic field capable of lifting at least ten pounds. Applicable inspection specifications require that the AC magnetic field generated by the inspection device have a strength sufficient to lift that amount of weight.
What is needed, then, is an electromagnetic particle inspection yoke having a battery-powered circuit that produces a true square wave voltage to provide highly reliable detection of defects in inspected parts. The needed yoke should generate a magnetic field strength capable of lifting at least ten pounds and should be capable of leaving the inspected part in a demagnetized condition.
However, it was not obvious to those of ordinary skill in this art how the needed yoke could be provided, in view of the art considered as a whole at the time the present invention was made.